The present invention relates to a reactor. More particularly, the present invention relates to a reactor having a fuel assembly which is excellent in the safety, the resistance to earthquake, the stability, the fuel soundness and the fuel economy.
Since commercial boiling water reactors were put into practical use, the latice structure of fuel rods of the fuel assembly have been changed from the 7-lines/7-columns structure to the 8-lines/8-columns structure, but the amount of uranium charged in the fuel assembly has not substantially been changed. The reason is that the characteristics of the core depend greatly on the value of the water-to-uranium ratio, and in making a partial change of the design, it is safer not to substantially change the amount of uranium charged in the fuel assembly. As a means for improving the fuel soundness, there has recently been proposed a method in which a hole having a diameter of about 3 mm is formed at the center of a fuel pellet. Also in this method, the change of the power factor is maintained at the conventional level by adjusting the size of the hole so that the change of the amount of uranium is within 10%.
The thought concerning the safety of a light-water reactor has been changed little by little for these twenty years. At the initial stage of the development, it was considered that the safety would be higher with an increase of the negative power factor administering the power self-control of automatically reducing the power when the power is increased by some cause or other and the amount of voids in a moderator is increased, and reactors were designed under this idea. However, to the compressed cold water transient phenomenon crushing voids, such as closing of a main vapor isolation valve or loss of feed water, too large a negative power factor results in reduction of the stability.
At the present, the re-processing cost is larger than the cost of enrichment of uranium, and charging of a large quantity of uranium-238 not participating directly in fission in the core and transfer of this core to a reprocessing plant will result in increase of the fuel cycle cost, and the weight of the core will be unnecessarily increased and the resistance of earthquake will be reduced.